CAPACIDAD DE CARGA REAL (CCR)

In this section, component sizing and the performance are compared when a RHP is designed for a given nominal capacity with and without a liquid receiver. In the analysis, it is assumed that when receiver is present the condenser outlet is saturated (free liquid draining from condenser to receiver) and there is no internal (eg. subcooling coil) or external (with environment) heat transfer. Further, it is assumed that there is no pressure drop in the liquid line, and the refrigerant in the receiver will also be saturated at the condensing pressure [Keller, 1999].

00 0.7 ■ D e s i g n m o d e □ R e v e r s e d m o d e .0 1.9 R407C 2 7 1 4 0.7 1.0 1.9 R 1 3 4 a 2 7 3 ^ v o l u m e ratio

Fig. 4.31 Refrigerant charge requirements in heating and cooling modes with RCI

a) Influence of receiver on com ponent sizing and heating perform ance

Fig. 4.32 shows the changes of the heating COP and the compressor size requirement when a RFIP (incorporated a LR) is designed at different Vr.

The results show that generally the compressor size is larger when the design includes a receiver (i.e. no subcooling available), and the increase (over a system without receiver) is higher for R407C than for R134a. Further, the influence of the receiver appears to reduce the COP, the degree of which is also higher for R407C system. The gradual decrease of compressor size requirement with Vr is attributed to the increase of evaporator pressure relative to the corresponding system without receiver. The reductions in COP with Vr are due to unavailability of condenser subcooling and larger compressor sizes.

q c_ o 10 O o 5 ■ o - 00 « - 1 0 -o !64 14.4 12.3 1 0 . 0 8.4 4.1 _3.7 -10. 2 -9 -12.5 -13.7 0 .7 1.0 1.9 2.7 3.4 S R 4 0 7 C I 1 R 1 3 4 a 2.7 2.2 _1.5 -4, 7 -3.5 -2. 9 -2. 7 -2.4 0 .7 1.0 1.9 2 . 7 3.5 v o lu m e ratio

Fig. 4.32 Influence of a receiver on the compressor size and heating COP: percentage changes relative to system without a receiver

Without any subcooling it is necessary that the refrigerant mass flow rate increase to maintain a given capacity. Within the range of Vr, the average increases in the mass flow rates are about 5% and 18% for

R I 34a and R407C respectively when compared to a system designed for a 5 °C subcooling (i.e. without receiver). The need for larger compressor sizes for R407C is due to larger refrigerant mass flow rate and the effect of glide, which changes the evaporating pressure when the inlet vapour quality changes (the opposite effect of having a SLHX, Payne et ah,

2000).

• c h a r g e r e q u ir e m e n t w ith a receiver

Table 4.10 shows that systems designed with and without a receiver need different amounts of charge, e.g. at Vr = 0.7, 4.13 kg compared to 3.78 kg, which becomes 4.39 kg, the actual system charge when a 1/6^^ is added. As given within brackets, the estimated charge requirement (i.e. without considering the added 1/6^ quantity) is lower when a receiver is present mainly due to the absence of subcooled refrigerant in the condenser.

T a b le 4.10 System charge requirements in heating mode __________________ with and without a receiver______________

System charge R407C System charge R134a

Volume ratio ______________ (^g)

W ithout receiver

W ith receiver W ithout receiver W ith receiver 0.7 4.13 4.39 (3.78) 3.46 3.85 (3.23) 1.0 T56 3.79 (3.27) 2.91 3.16(2.73) 1.9 • 3.13 3.31(2.85) 2.45 2.73 (2.35) 2.7 2.97 3.17(2.73) 2.30 2.47 (2.14) 3.4 Z87 3.05 (2.63) 2.20 2.36 (2.03) 155

• C irculating compositions and heating perform ance

Changes in the system charge when a receiver is present, as given in Table 4.10, influence the circulating mixture composition. Fig. 4.33 presents the running R407C compositions for five Vrs designed with a liquid receiver. Systems with a receiver show a similar trend of change of composition as those without a receiver (Fig. 4.23); however, the magnitudes o f shift are relatively higher. Further enrichment of circulating mixture with R32 and R125 occurs due to the hold up of liquid R134a in the receiver. However, the shift in the circulating composition varies with the amount of extra refrigerant added (normally

16%) to maintain the liquid seal.

2 0 . 0 -I □ R32 ■ R 125 E R 1 3 4 a 2 10.0 R -10.0 -20.0 - 3 0 . 0 J 0 .7 1.9 2 . 7 3 .4 v o l u m e ratio

Fig. 4.33 Running mixture concentrations when a receiver is present, percentage change relative to original R407C composition (23/25/52%

R32/R125/R134a)

Fig. 4.34 presents the heating capacities and the COPs when the composition shifts are taken into consideration for both systems with and without a receiver. As expected, the running capacities are higher for the

former and the difference between the two capacities are larger at low Vr due to larger shifts. However, the COP of a system with receiver is lower than that of a system without LR.

The analysis shows that the refrigerant mass flow rates are relatively higher for system with a receiver, about 20% to 30% compared to a system without one. The above increases in the heating capacity are due to the improvements in the heat transfer caused by increase of refrigerant mass flow rate. The reduction in the COP is due to relative increase in the compressor work resulted from increased mass flow rate and the composition shift. — w i t h r e c e i v e r w i t h o u t r e c e i v e r O c a p a c i t y A C O P 10.0 1 C L O U o 4 . 0 1.25 2 .2 5 3 .2 5 0 . 2 5 v o l u m e ratio

Fig. 4.34 Heating performance at running compositions with and without a receiver (nominal capacity 7.5 kW)

b) C ir c u la tin g co m p o sitio n and p e r fo r m a n c e d u rin g coolin g

Table 4.11 presents the running compositions of R407C system in the cooling mode and the amount of refrigerant in the receiver when reversed using RCl and RC2. At Vr = 0.7 and 1, the receiver contains

only about the added 1/6^^ of refrigerant. However, for Vr larger than 1, the amount held up in the receiver increases with Vr. With both reversing methods, on the contrary to the case of the system without a receiver (Table 4.8) the positive shift in circulating composition therefore increases with Vr. The composition shifts with RC2 are slightly lower than those of RCl at each Vr. This is due to relatively a larger amount of charge retained in the heat exchangers with RC2 than with RCl by about 5% to 10% compared to those with R C l.

T able 4.11 Circulating mixture composition and amount of charge in receiver in cooling mode

Vr Circulating mixture composition

R C l RC2 R32 R125 R134a charge in LR (kg) R32 R125 R134a charge in LR(kg) 0.7 0T22 0.312 0366 0.73 0.302 0.292 0.406 0.68 1.0 0.375 0.366 0.259 032 0.355 0.346 0.299 0.50 1.9 0 J8 7 0380 0.233 0.66 0.367 0.360 0.273 0.63 2.7 0.391 0.384 0.225 0.79 0.371 0.364 0.265 0.72 3.4 0 J9 4 0389 0.217 0.91 0.374 0369 0.257 036

Fig. 4.35 presents the cooling performance o f R407C system using R C l, relative to the systems designed without a receiver. With a receiver, the cooling COP dropped by about 8% to 16% whereas the capacities show improvements of over 10%. The drops in COP are mainly due to the use of a relatively large compressor in each system, and the increase in pressure ratio with Vr (from 3.1 at Vr = 0.7 to 3.9 at Vr = 3.4). Increase in the capacity can be attributed to the combined effect of using larger

compressors (compared to the system without a receiver) and the increase in positive shift.

C L c o o ^ 0 J 5 0 . 5 0 ■ c a p a c i t y □ C O P 0.7 1.9 2.7 3. 4 v o l u m e ratio

Fig. 4.35 Relative cooling performance: R407C system

On the other hand, the analysis of the R134a system, Fig. 4.36, shows that the influence of the receiver on the performance is negligible. This is due to relative smaller loss in specific refrigerating effect in the absence of both the effects of glide and the composition shift. Therefore, for pure fluid systems, the decision of including a liquid receiver can be based on the storage requirement for pump down cycle. However, it must be pointed out that the limitation of liquid back up conditions discussed earlier also applied. Therefore Figs. 4.35 and 4.36 can be compared only in relative sense.

C- C O o > 0 .7 5 - 0 . 5 0 0.7 1.9 2.7 ■ c a p a c i t y □ C O P 3.4 v o l u m e ratio

Fig. 4.36 Relative cooling performance: R134a system c) Bypassing liquid receiver

An exercise was carried out to evaluate the possibility of by-passing the receiver during operation of the mode when there is no excess charge, i.e. the cooling mode for Vr < 1.0 and the heating mode for Vr > 1.0 for the system discussed in previous section. When the receiver is bypassed the operating conditions and the amount of charge available for circulation lead to a certain degree of condenser subcooling that could enhance the system performance to some extent (provided that the amount of subcool does not lead to liquid back up). Table 4.12 shows the potential performance gain when the receiver is bypassed (at selected Vr); and assumed to be completely empty.

Table 4.12 Performance when receiver is on line and offline: R407C systems

Receiver Vr = 0.7 (cooling m ode) Vr = 3.4 (heating m ode)

option _{CO P Capacity (kW ) C O P Capacity (kW )}

On line 5.12 6.14 1 3 1 1 9 9

However, practically a certain amount of liquid refrigerant would stay in the receiver. The amount would depend on the receiver design and the type of heat exchanger. Fig. 4.37 shows the relative changes in capacity and COP for Vr = 3.4 considering different amounts of refrigerant remained in the receiver when bypassed.

O C a p a c i t y A C O P O 1 u o O cS Cl C3 CJ (Ü > 1.05 - a 1.00 4. 0 8.0 1 2 .0 16.0 % c h a r g e in r e c e i v e r

Fig. 4.37 Influence of the amount of charge staying in the receiver on potential performance gain of receiver bypassing

4.4 Passive capacity control

When using refrigerant mixtures, passive capacity control techniques can be applied to vary the capacity of single direction heat pumps (effectively changing the circulating composition) when the source temperature changes. In the present study, the emphasis is on the possibility of using the excess refrigerant charge occurring in reversible systems to obtain capacity control. When the idea of a two accumulator cycle is presented. Vakil and Flock (1980) did not consider the changes in the mixture composition due to the HPA, i.e. receiver, (the running

composition was considered to be the same as the charged composition when the refrigerant is in the HPA).

However, as presented in the previous section, the running composition is in fact different to that of the original mixture when the excess refrigerant is in the receiver (i.e. HPA). Therefore, the present analysis uses the circulating composition as appropriate to obtain the system performance. First an estimate of the possible capacity increase or decrease in the heating (designed) mode of a RHP is investigated when using a two-accumulator configuration (Fig. 2.10). This is followed by an investigation of how the composition change associated with the two- accumulator system could be used when using a four-way valve (i.e. with R C 1), for different Vr.

• Designed mode

A drop, say 6 °C, in water return temperature to the evaporator, (compared to those specified at the design) results in about 17% and 7% drops in heating (designed) capacity and COP respectively of a R407C system o f Vr = 0.7. At this instance, about 30% (i.e. 1.44 kg) of the total system charge stays as excess charge in the receiver (Note: at Vr = 0.7, the normal excess charge in the design condition is 1.35kg).

Assuming the LPA is of sufficient volume, if the total available excess charge were transferred from HPA to LPA (as Vakil and Flock (1980) suggested), the situation corresponds to a system with a saturated evaporator outlet, with the compressor drawing vapour rich in lower

boiling components (R32 and R125). The circulating compositions of R32 and R125 change from 0.304 and 0.292 (when excess charge is in HPA), to 0.358 and 0.341 when transferred into LPA. As given in Fig. 4.38, this change in composition increases the capacity by about 11.5%, with a small reduction in the COP compared to when the total amount of excess charge remains in the HPA (i.e. the liquid receiver). This represents a 93% of the original design running capacity (i.e. 8.06 kW).

CL o u 9- 6.5 □ E x c e s s c h a r g e in H P A ■ E x c e s s c h a r g e In L P A R u n n i n g c a p a c i t y ( d e s i g n m o d e ) D e s i g n e d c a p a c i t y c a p a c i t y C O P

Fig. 4.38 Heating performance with the excess charge in LPA and/or in HPA : R407C, Vr = 0.7

Nonetheless, a suitable mechanism should be made available to transfer the liquid refrigerant from HPA to LPA, and the LEV control needs further consideration. However, Vakil and Flock (1980) did not address these practical aspects of HPs. Further, if the liquid seal in the receiver were to maintain the total amount of excess may not be transferable to

LPA. The analysis in this case only allows 14% of the excess charge to be transferred to LPA, the increase in capacity is therefore smaller; about 7.0%.

The control strategy is useful to change the capacity between the two limits decided by the two concentrations at a particular water return temperature. The lower capacity corresponds to the circulating concentration when all the excess liquid is in the receiver (HPA), and the upper value corresponds to a situation where the transferable excess charge is "moved" to LPA (subjected to the requirements o f liquid seal in HPA). By controlling the amount of refrigerant transferred to the LPA, intermediate capacities within the two limits can be obtained.

• R e v e r s e d m o d e

For analysis purposes, the two-accumulator cycle can be considered as a system consisting of a condenser and an evaporator each carrying a receiver (or a small vessel) at the refrigerant exit end (with appropriate inlet/outlet pipe connections in each vessel). This allows applying the above concept of transferring the refrigerant between the LPA and the HPA in RHPs to vary the circulating composition, but only in the mode where excess charge occurs and this is a function of Vr.

However, a certain degree of capacity control can be achieved if the system was charged with extra refrigerant at the design (i.e. with a view of also obtaining capacity control in the reversed mode). This additional charge represents the transferable amount between the two liquid

vessels; naturally this would mean that the two capacity limits previously discussed would change. Fig. 4.39 presents the changes in the amount of R32 in the circulating mixture, COP and capacity when different amounts of extra charge is added to the system relative to design charge.

The potential of increasing the cooling capacity increases with the amount of added charge, e.g. about 5% increase in cooling capacity if the system was charge with 11 % extra refrigerant than the designed charge (Fig. 4.39). Due to the effect o f positive shift, the COP decreases when the amount of added charge increases. To achieve a larger change in capacity, relatively larger amounts of liquid need to be stored in the LPA [Rothfleisch, 1995]. However, the increase in amount of charge in the system invariably changes the circulating composition also in the forward mode, further enriching the circulating composition with R32 and R125. charge ( r e l a t i v e t o d e s i g n m o d e c h a r g e ) □ R32 fraction capacity 1 , 0 6 1 , 0 5 01) 0 . 6 5 1.34 Charge in L P A / kg 1.78

Fig. 4.39 Increase of R32 fraction and changes of the capacity and the COP with added charge (relative to design mode)